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The Oligocene–Miocene Kailas Formation is exposed along strike for ∼1300 km within the southernmost Lhasa terrane. In this study, we documented the sedimentology, structure, and age of this unit exposed between 87°E and 90°E. Within this region, the Kailas Formation is composed of continental deposits dominated by conglomerate and sandstone, with lesser volumes of siltstone and paleosols. These rocks were deposited nonconformably on Gangdese Batholith and related volcanic rocks along their northern boundary, whereas to the south, the south-dipping Great Counter Thrust places them in contact with Xigaze forearc and melange units. We interpret the Kailas Formation to have been deposited in alluvial-fan and fluvial environments with sediment principally derived from the north. Based on sedimentology and structural relationships, we interpret these rocks to have formed in a north-south extensional setting. New zircon U-Pb ages from volcanic tuffs and flows show that Kailas Formation deposition is younger to the east: Deposition occurred between 26 Ma and 24 Ma in western Tibet (81°E), at 25–23 Ma north of Lazi (87.8°E), at 23–22 Ma near Dazhuka (89.8°E), and as late as 18 Ma southwest of Lhasa (92°E). Overall, basin development propagated eastward at a rate of ∼300 mm/yr. This pattern and rate of propagation are similar to that of the temporal-spatial distribution of adakitic and ultrapotassic magmatism within the Lhasa terrane to the north, which has been interpreted as a record of slab breakoff. Magmatism lags several million years behind Kailas basin development at most locations. We interpret the Kailas basin to have formed as the result of Indian slab shearing and breakoff, which began in western Tibet around 26 Ma and reached eastern Tibet by ca. 18 Ma.

Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. We investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Our results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55–65-km-thick crust at 85–95 Ma. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30–40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems.

The link between orogenic activity and foreland basin stratigraphy is well established; however, potential controls by foreland basin stratigraphy on thrust belt architecture have not been fully evaluated. Mechanical properties of typical foreland basin stratigraphic successions influence the structural development of fold-thrust belts in predictable ways. Fundamental features of foreland basins include the onset of rapid subsidence and deposition of a coarsening-upward sedimentary succession. In the lower part of this succession are fine-grained, distal foreland basin deposits. Enlargement of the orogenic wedge through frontal accretion incorporates the foreland basin strata into the thrust belt, and distal foreland basin depositional units may be preferentially exploited as a thrust detachment zone, resulting in multiple detachment levels. We propose that foreland basin stratigraphic architecture has significant influence on the structural development of thrust belts and that, by extension, processes that influence foreland basin sedimentation may ultimately influence orogenic evolution far removed in time and space.

Siliciclastic sedimentary rocks derived from the southern Lhasa terrane, sitting depositionally upon rocks of the northern Indian passive continental margin, provide an estimate of the age of initial contact between the continental parts of the Indian and Asian plates. We report sedimentological, sedimentary petrological, and geochronological data from Upper Cretaceous-Paleocene strata in the Sangdanlin section, located along the southern flank of the Indus-Yarlung suture zone in southern Tibet. This is probably the most proximal, and therefore the oldest, record of the India-Asia collision. These strata were deposited by high-density turbidity currents (or concentrated density flows) and suspension settling of pelagic biogenic debris in a deep-marine setting. An abrupt change from quartz-arenitic to feldspatholithic sandstone compositions marks the transition from Indian to Asian sediment provenance. The abrupt compositional change is accompanied by changes in U-Pb ages of detrital zircons diagnostic of a sediment provenance reversal, from Indian to Asian sources. The timing of the transition is bracketed between ~60 Ma and 58.5 ± 0.6 Ma by detrital zircon U-Pb ages and zircon U-Pb ages from a tuffaceous bed in the upper part of the section. In the context of a palinspastically restored regional paleogeographic framework, data from the Sangdanlin section combined with previously published data from the northern Tethyan Himalaya and the frontal Nepalese Lesser Himalaya and Subhimalaya suggest that a flexural wave migrated ~1300 km southward across what is now the Himalayan thrust belt from Paleocene time to the present.

The India-Asia collision zone in southern Tibet preserves a record of geodynamic and erosional processes following intercontinental collision. Apatite fission-track and zircon and apatite (U-Th)/He data from the Oligocene–Miocene Kailas Formation, within the India-Asia collision zone, show a synchronous cooling signal at 17 ± 1 Ma, which is younger than the ca. 26–21 Ma depositional age of the Kailas Formation, constrained by U-Pb and 40 Ar/ 39 Ar geochronology, and requires heating (burial) after ca. 21 Ma and subsequent rapid exhumation. Data from the Gangdese batholith underlying the Kailas Formation also indicate Miocene exhumation. The thermal history of the Kailas Formation is consistent with rapid subsidence during a short-lived phase of early Miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the Indian plate, respectively. Significant removal of material from the India-Asia collision zone was likely facilitated by efficient incision of the paleo–Indus River and paleo–Yarlung River in response to drainage reorganization and/or intensification of the Asian monsoon.

International audience; The tectonic and topographic history of the Himalaya-Tibet orogenic system remains controversial, with several competing models that predict different exhumation histories. Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate asymmetric exhumation of Mount Everest consistent with a scenario in which the southern edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. Northward plateau retreat was caused by erosional incision during the Pliocene. Our results suggest that the South Tibetan Detachment was a localized structure and that no coupling between precipitation and erosion is required for Miocene exhumation of Greater Himalayan Sequence rocks on Mount Everest.

Cordilleran orogenic systems such as the Andes are controlled by shortening rates, climatically-controlled erosion rates, and, in some cases, eclogite production and delamination. All of these processes are coupled, however, making it diffi cult to uniquely determine the relative importance of each process and the feedbacks among them. In this paper we develop a massbalanced numerical model that couples an actively-shortening orogen and crustal root with eclogite production, delamination, and climatically controlled erosion. The model provides a fi rst-order quantifi cation of the sources (shortening) and sinks (erosion and eclogite production and delamination) of crustal volume during the Cenozoic in the Andes as a function of latitude and time. Given reasonable estimates for the rates of eclogite production and the threshold size of the eclogitic root required for delamination, the model suggests that, in the central Andes between 5° S and 32° S, the orogen has grown to a suffi cient height to produce and maintain eclogite, which in turn has promoted delamination in the lower crust and mantle. In this region, climatically controlled erosion rates infl uence the size of the orogen through two separate mechanisms: by exporting mass via surface processes and by controlling the lithostatic pressure in the lower crust, which modulates the rates of eclogite production and/or delamination. To the north and south of the central Andes, relatively low shortening rates and high precipitation and erosion rates have slowed eclogite production such that delamination likely has not occurred during the Cenozoic.

Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks. This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust. Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata, juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about 15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades, although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic metamorphism.

A geological and geochronologic investigation of the Nima area along the Jurassic–Early Cretaceous Bangong suture of central Tibet (∼32°N, ∼87°E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110–106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic–Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous–Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and N-directed thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N–S shortening following Early Cretaceous ocean closure and ∼25 km shortening (∼28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system.

We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of the MCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100 ± 3400 t/km2 and 5.1 ± 1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6 ± 0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of the Greater Himalaya, where rainfall decreases rapidly to b 2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4 ± 0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to b 0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are ∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the central Nepal eight-thousanders may be viewed as topographic anomalies in cold desert climate at the southern edge of the Tibetan rain shadow. © 2007 Elsevier B.V. All rights reserved.

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 188 and 208 S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by ~70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After ~20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (~8–10 mm/year) through time with possible slowing (~5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination. D 2005 Elsevier B.V. All rights reserved.

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western U.S.A. In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time ( 155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate. Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges. At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the "Nevadaplano"); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins. Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward. By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt. The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic mega- thrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleo- zoic and Mesozoic thrust sheets during Late Cretaceous-Paleocene time. Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean- Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting. A complex foreland basin system evolved in concert with the orogenic wedge. During its early and late history ( 155 - 110 Ma and 70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife ( 110 - 70 Ma). Late Jurassic basin development was controlled by both flexural and dynamic subsidence. From Early Cretaceous through early Late Creta- ceous time the basin was dominated by flexural subsidence. From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures. Linkages between Late Jurassic and Late Cretaceous Cordilleran arc- magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt. A significant lag-time ( 20 Myr) between shortening and coeval underthrust- ing, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist. However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continen- tal underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up.

East-west extensional basins are distributed across the southern half of the Tibetan plateau at an elevation of ∼4 km. These basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau. This study documents the sedimentology of the Miocene–Pliocene Thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment. Analysis of depositional systems, paleodrainage patterns, and conglomerate clast provenance of the >1-km-thick graben fill sets limits on the timing of activity of the basin-bounding faults and the development of southward axial drainage in the basin. During the deposition of the oldest basin fill (Tetang Formation, ca. 11–9.6 Ma), probably in a restricted basin, minor motion occurred on the basin-bounding fault systems. An angular unconformity separates the Tetang and overlying Thakkhola Formations, where this contact can be observed in the southern part of the basin. Southward axial drainage was established by ca. 7 Ma with the onset of deposition of the Thakkhola Formation. Several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin. Facies distribution and the progressive rotation of strata in the Thakkhola Formation indicate that the Dangardzong fault on the western edge of the basin was active at this time, and drainage ponding may have been related to displacement on normal faults associated with the South Tibetan detachment system to the south of Thakkhola graben. Contrasts between deposits of the Tetang and Thakkhola Formations provide evidence for environmental change in the basin. In the Tetang Formation, the abundance of lacustrine facies, the pollen record, and the absence of paleosol carbonate suggest that conditions were more humid than during subsequent deposition of the Thakkhola Formation. Environmental change in the Thakkhola graben coincided with environmental change observed in the Siwalik foreland basin sequence, Arabian Sea, and Bay of Bengal at ca. 8–7 Ma. Although this climate change event has been previously attributed to intensification of the Asian monsoon in response to uplift of the Tibetan plateau, paleoaltimetry data indicate that this region had already attained a high elevation by ca. 11 Ma. Thus, the Thakkhola graben stratigraphic record suggests that uplift of the southern Tibetan plateau and the onset of the Asian monsoon as inferred from paleoclimatic indicators were not directly related in a simple way.

The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern. Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 Ar-40/Ar-39 ages from detrital minerals derived from rivers in China draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern, central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some affinity with the Qiangtang terrane of central Tibet. The epsilon(Hf) values are juvenile, between 15 and -5, for the northeastern Pamir and juvenile to moderately evolved, between 10 and -40, for the central and southern Pamir. Detrital mica Ar-40/Ar-39 ages for the northeastern Pamir (eastern drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene-Pliocene signals at the orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes, average erosion rates for the northeastern Pamir are up to one order of magnitude lower than erosion rates recorded by the central and southern Pamir. Deeper exhumation of the central and southern Pamir is associated with tectonic exhumation of central Pamir domes. Deeper exhumation coincides with western and asymmetric drainages and with higher precipitation today, suggesting an orographic effect on exhumation. A younging-southward trend of cooling ages may reflect tectonic processes. Overall, cooling ages derived from the Pamir are younger than ages recorded in Tibet, indicating younger and higher magnitudes of erosion in the Pamir.

Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium isotopic data from the Himalayan fold-thrust belt in Nepal (n=35) reveal that the Lesser Himalayan zone consistently has a more negative ϵNd(0) value than the Greater and Tibetan Himalayan zones. Our data show the average ϵNd(0) value in the Lesser Himalayan zone is −21.5, whereas the Greater and Tibetan Himalayan zones have an average ϵNd(0) value of −16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative ϵNd(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits (n=34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The ϵNd(T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones. At ∼10 Ma in Khutia Khola and ∼11 Ma in Surai Khola, a shift in ϵNd(T) values from −16 to −18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the ϵNd(T) values remain close to −16 throughout Miocene time because there has been less erosional unroofing in this region.

Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland-basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800-22 600 km2), fan-shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low-relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid-channel and bank-attached sand bars. Overbank areas are characterized by crevasse-splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long-term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening-upward, >2-km-thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold-thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid-Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse-splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold-thrust belt. In addition, the present-day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid-Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold-thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 104 km2 ± a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold-thrust belt.

The δ18O value of meteoric water varies with elevation, providing a means to reconstruct paleoelevation if the δ18O values of paleowater are known. In this study, we determined the δ18O values of water (δ18Omw) from small tributaries along the Seti River and Kali Gandaki in the Nepal Himalaya. We found that δ18Omw values decrease with increasing altitude for both transects. δ18Omw vs. altitude along the Kali Gandaki in west-central Nepal fit a second order polynomial curve, consistent with increasing depletion of 18O with increasing elevation, as predicted by a Rayleigh-type fractionation process. This modern δ18Omw vs. altitude relationship can be used to constrain paleoelevation from the δ18O values of Miocene–Pliocene carbonate (δ18Oc) deposited in the Thakkhola graben in the southern Tibetan Plateau. Paleoelevations of 3800±480 m to 5900±350 are predicted for the older Tetang Formation and 4500±430 m to 6300±330 m for the younger Thakkhola Formation. These paleoelevation estimates suggest that by ∼11 Ma the southern Tibetan Plateau was at a similar elevation to modern.

Our study of the Seti River in far western Nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology. The Seti flows through four distinct terranes, starting with the Tethyan sedimentary series (TSS) and Greater Himalayan series (GHS). TSS/GHS waters display 87 Sr/ 86 Sr ratios of ,0.73 and high Sr and Ca, consistent with the composition of limestone and marble common in these terranes. The 87 Sr/ 86 Sr ratio and Mg increase markedly as the river passes into the Lesser Himalayan series (LHS), where tributaries have 87 Sr/ 86 Sr ratios from 0.75 to 1.02 and high Sr, Ca, and Mg. The high Mg in LHS waters correlate with high 87 Sr/ 86 Sr ratios, which we attribute to weathering of highly radiogenic (0.71- 0.82) dolostones. Tributaries to the Seti River draining the largely carbonate-free Dadeldhura thrust sheet (DTS) have ratios near 0.74, but low Sr, Ca, and Mg and therefore have little impact on Seti mainstem chemistry. Mass balance calculations and CaMg-weathering indices show that carbonate weathering accounts for .70% of total dissolved solids to the Seti River. Sr/Ca ratios of river waters provide a minimum estimate of the %-carbonate weathering contribution to Sr, due to partitioning of Sr and Ca during incongruent dissolution and reprecipitation of calcite. Overall, we attribute high 87 Sr/ 86 Sr ratios in the Seti River and its tributaries to the weathering of metacarbonates (especially dolostones in the upper Nawakhot Group) which have exchanged Sr with silicates during metamorphism. Our modeling of Sr fluxes in the Seti River indicates that the TSS/GHS accounts for 36 -39% of the Sr, the LHS for 40 -53%, and 8 -23% for the DTS. Prior to exposure of LHS rocks at ;12 Ma, TSS and GHS carbonates with low 87 Sr/ 86 Sr ratios dominated Himalayan rivers. We attribute the elevated 87 Sr/ 86 Sr ratios of Himalayan paleorivers during the late Miocene and Pliocene to exposure and weathering of LHS metacarbonates. Copyright © 2000 Elsevier Science Ltd

East-west extension in the Tibetan Plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift. On the basis of this assumption, several studies have dated east-west extensional structures to determine when the plateau attained its current high elevation. However, independent estimates of elevation are needed to determine whether extension occurred before, during, or after the plateau achieved its current elevation. Because the isotopic composition of meteoric water decreases with increasing elevation, significant change in local elevation throughout the Thakkhola graben depositional history should be recorded by change in δ 18 O values of fluvial and lacustrine carbonates. The δ 18 O values of ‐16‰ to ‐23‰ of Thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern Tibetan Plateau attained its current elevation prior to eastwest extension. Initiation of Thakkhola graben extension is constrained between 10 and 11 Ma, based on magnetostratigraphy of the older Tetang Formation. The δ 13 C values of soil carbonates suggest an age younger than 8 Ma for the base of the Thakkhola Formation.

The Campanian–Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern Wyoming consists of a widespread (>10 000 km 2 ) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward–flowing, gravelly braided rivers on top of the frontal part of the Sevier fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland basin. In northeastern Utah the conglomerate was deposited in a lobate fan-shaped body, up to 122 m thick, that trends southeastward away from its principal source terrane in the southern end of the Willard thrust sheet. The Willard sheet contains thick Proterozoic quartzite units that produced highly durable clasts capable of surviving long-distance fluvial transport. Although the main source of sediment for the Hams Fork Conglomerate was the Willard sheet, the active front of the thrust belt lay 40–50 km to the east along the Absaroka thrust system. Displacement along the Absaroka system uplifted and topographically rejuvenated the Willard sheet, and antecedent drainages carried detritus from hinterland source terranes into the proximal foreland basin. Although topographic ridges associated with fault-propagation anticlines along frontal thrusts locally influenced transport directions, they provided relatively little sediment to the Hams Fork Conglomerate. Lithofacies, paleocurrent, and isopach data indicate that the Hams Fork Conglomerate was deposited in fluvial megafans and stream-dominated alluvial fans, similar in scale and processes to megafans and alluvial fans in southern Nepal and northern India that are forming along the proximal side of the Himalayan foreland basin system. The Himalayan fluvial megafans have areas of 10 3 –10 4 km 2 , slopes of 0.05°–0.18°, and are deposited by large transverse rivers that are antecedent to frontal Himalayan structures and topography. The main fluvial channels on the upper parts of the megafans are anastomosed and braided at bankfull stage but commonly have braided thalwegs at low-flow stage. Downstream, these channels become predominantly braided and meandering and ultimately merge with the axial Ganges trunk river system. Stream-dominated alluvial fans in the Himalayan foreland basin system fringe the topographic front of the fold-thrust belt in the intermegafan areas. These fans have areas of ∼10 2 km 2 and slopes of ∼0.5°. The proximal parts of both types of fans are dominated by extremely coarse (boulder-cobble) bedload that is in transit mainly during the monsoon. The prevalence of fluvial megafans in the modern and Miocene Himalayan foreland and in the Upper Cretaceous–lower Tertiary stratigraphic record of the Cordilleran foreland suggests that these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems.

The Messinian stratigraphy of eastern Calabria (southern Italy) is characterised by a threefold subdivision: (1) a pelite section with local limestone and gypsum, deposited in a restricted-marine environment, is unconformably, or disconformably, overlain by (2) coarse-grained alluvial conglomerate, which is in turn locally overlain by (3) a thin and discontinuous ribbon-shaped sedimentary body of sandstone and pelite, commonly displaying a shallow-marine to continental progradational trend. The basal unconformity/disconformity, coarse grain-size, and abrupt compositional-sedimentological change of unit 2 with respect to unit 1 can be explained as a response to tectonic instability and out-of-sequence thrusting in the Calabrian orogenic wedge, possibly induced by isostatic back-tilting of the wedge following the desiccation of the Mediterranean Sea. This mechanism could explain widespread late Messinian thrusting and syntectonic sedimentation along the Apenninic–Maghrebian orogenic belt. The uppermost Messinian continental to shallow-marine siliciclastic deposits of unit 3 crop out today at elevations of up to 300 m. Similar, age-equivalent sedimentary deposits can be traced along the Apennines and the Sicilian Maghrebides, thus, indicating that the Mediterranean area was flooded before deposition of the Trubi Formation, the base of which is traditionally regarded as marking the reestablishment of marine conditions in the Mediterranean region.

Sedimentologic, petrographic, and U-Pb detrital zircon ages from middle Eocene through early Miocene sedimentary rocks in the Lesser Himalayan zone of western and central Nepal indicate that a peripheral foreland basin system had developed in the eastern Himalayan collision zone by middle Eocene time. The shallow-marine, Eocene Bhainskati Formation accumulated in a back-bulge depozone between a southward migrating forebulge and the Indian craton. Migration of the forebulge through this region during Eocene-Oligocene time produced a regional unconformity that spans ∼15–20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone, represented by fluvial deposits of the Dumri Formation. Continued southward migration of the foredeep during the Neogene accommodated the fluvial Siwalik Group. Light mineral provenance data and U-Pb detrital zircon ages suggest that the Bhainskati was derived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zone during initial growth of the Himalayan fold-thrust belt. The Dumri was derived from metasedimentary and crystalline rocks of the Greater Himalayan zone during emplacement of the Main Central thrust and contemporaneous tectonic unroofing by normal faulting along the South Tibetan detachment system. The Lesser Himalayan crystalline thrust sheets were emplaced soon after deposition of the Dumri Formation, ∼15–10 Ma. Paleocurrent and lithofacies data from the Dumri Formation indicate deposition by west-southwestward flowing rivers that drained into the Indus portion of the Himalayan foreland basin system during the early Miocene. Thick channel sandstones in the lower Dumri may represent the early Miocene counterpart of the modern Ganges River. Eastward diversion of the Ganges drainage system to near its present location had occurred by ∼15 Ma, as the high-standing Aravalli Range on the northern Indian shield approached the front of the fold-thrust belt. Assuming reasonable values for the flexural rigidity of Indian lithosphere, the time span of the forebulge unconformity yields a velocity of ∼14–33 mm/yr for the southward migration of the fold-thrust belt relative to India. This range of values is consistent with Neogene and present-day estimates and suggests that only one third to one half of India-Eurasia convergence has been accommodated by shortening in the Himalayan fold-thrust belt since the onset of collision.

A foreland basin system is defined as: (a) an elongate region of potential sediment accommodation that forms on continental crust between a contractional orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold-thrust belt; (b) it consists of four discrete depozones, referred to as the wedge-top, foredeep, forebulge and back-bulge depozones – which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt; (c) the longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens). The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins. Wedge-top sediment tapers toward the hinterland and is characterized by extreme coarseness, numerous tectonic unconformities and progressive deformation. The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. This sediment typically thickens rapidly toward the front of the thrust belt, where it joins the distal end of the wedge-top depozone. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back-bulge depozones. The back-bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge. This more inclusive definition of a foreland basin system is more realistic than the popular conception of a foreland basin, which generally ignores large masses of sediment derived from the thrust belt that accumulate on top of the orogenic wedge and cratonward of the forebulge. The generally accepted definition of a foreland basin attributes sediment accommodation solely to flexural subsidence driven by the topographic load of the thrust belt and sediment loads in the foreland basin. Equally or more important in some foreland basin systems are the effects of subduction loads (in peripheral systems) and far-field subsidence in response to viscous coupling between subducted slabs and mantle–wedge material beneath the outboard part of the overlying continent (in retroarc systems). Wedge-top depozones accumulate under the competing influences of uplift due to forward propagation of the orogenic wedge and regional flexural subsidence under the load of the orogenic wedge and/or subsurface loads. Whereas most of the sediment accommodation in the foredeep depozone is a result of flexural subsidence due to topographic, sediment and subduction loads, many back-bulge depozones contain an order of magnitude thicker sediment fill than is predicted from flexure of reasonably rigid continental lithosphere. Sediment accommodation in back-bulge depozones may result mainly from aggradation up to an equilibrium drainage profile (in subaerial systems) or base level (in flooded systems). Forebulge depozones are commonly sites of unconformity development, condensation and stratal thinning, local fault-controlled depocentres, and, in marine systems, carbonate platform growth. Inclusion of the wedge-top depozone in the definition of a foreland basin system requires that stratigraphic models be geometrically parameterized as doubly tapered prisms in transverse cross-sections, rather than the typical ‘doorstop’ wedge shape that is used in most models. For the same reason, sequence stratigraphic models of foreland basin systems need to admit the possible development of type I unconformities on the proximal side of the system. The oft-ignored forebulge and back-bulge depozones contain abundant information about tectonic processes that occur on the scales of orogenic belt and subduction system.

] In our recent paper [van Hinsbergen et al., 2011a], weprovide a kinematic restoration of Cenozoic deformation inAsia based on the currently available kinematic estimates onfault zones and fold-thrust belts in Tibet, the Pamir, the TienShan, Mongolia, Siberia and Indochina. Our reconstructionsuggests that approximately 1050 km (in the Pamir) to600 km (in eastern Tibet) of India-Asia convergence wasaccommodated by intraAsian shortening in the last !50 Ma.By comparing this reconstruction with the respective posi-tions of India and Asia constrained by the Eurasia-NorthAmerica-Africa-India plate circuit (using model A of vanHinsbergen et al. [2011b]), we explored the implicationsfor the size for Greater India as a function of collision age,whereby we define Greater India as ‘the area of lithosphereconsumed by northward subduction beneath the Asianmargin since collision of the Tibetan Himalaya with Asia’.Importantly, wedonot aprioridefine thatall thatlithospheremust be continental in nature. Our reconstruction demon-strated that if collision started by 50 Ma, Greater India at thetime of initial collision must have been up to 2600 km wide.Such a 50 Ma collision age follows from the timing of thefirst arrival of Asia-derived detritus in the Tibetan Himalaya[Najman et al., 2010; Wang et al., 2011; Hu et al., 2012] andthe overlap of paleomagnetically determined paleolatitudesfrom the former southern margin of Asia (the Lhasa terrane)[e.g., Dupont-Nivet et al., 2010a; Lippert et al., 2011], withthose from the Tibetan Himalaya [Patzelt et al., 1996; Yiet al., 2011]. Such a large N-S width of Greater India inLate Cretaceous and Paleocene time is consistent with thehigh-quality paleomagnetic data from Tibetan Himalayanrocks of Patzelt et al. [1996], shown to have undergonenegligible inclination shallowing due to compaction byDupont-Nivet et al. [2010b], and recently corroborated by Yiet al. [2011]: When compared to high-quality paleomagneticpoles from India, those paleomagnetic data demonstrate aN-S separation between the Tibetan Himalaya and theIndian craton of 22.0 " 3.0

The four-part districting scheme (wedge-top, foredeep, forebulge, and backbulge depozones) applies to many foreland basin systems worldwide, but significant variations occur in the stratigraphic record. These variations depend on tectonic setting and the nature of the associated fold-thrust belt. Continued growth of the foldthrust belt by horizontal shortening requires foreland lithosphere to migrate toward the fold-thrust belt. The flexural wave set up by the topographic load may migrate � 1000km sideways through the foreland lithosphere, a distance that is comparable to the flexural wavelength. This extreme lateral mobility results in the vertical stacking of foreland basin depozones in the stratigraphic record. The standard stratigraphic succession consists of a several km-thick upward coarsening sequence, marked in its lower part by a zone of intense stratigraphic condensation or a major disconformity (owing to passage of the forebulge), and in its upper part by coarsegrained proximal facies with growth structures (the wedge-top depozone). Foredeep deposits always reside between the forebulge disconformity/condensation zone and wedge-top deposits, and backbulge deposits may be present in the lowermost part of the succession. Wedge-top deposits are vulnerable to erosion because of their high structural elevation, and preservation of backbulge and forebulge deposits depends in part on tectonic setting. Three main types of fold-thrust belt are recognized: retroarc, collisional (or peripheral), and those associated with retreating collisional subduction zones. Retroarc foreland basin systems (such as the modern Andean) are susceptible to far-field dynamic loading transmitted to the foreland lithosphere by viscous coupling between the subducting oceanic slab and the mantle wedge. This longwavelength subsidence adds to subsidence caused by the topographic flexural wave, allowing for preservation of well-developed forebulge and backbulge depozones. The absence of dynamic subsidence in collisional (peripheral) foreland basin systems (such as the modern Himalayan) renders forebulge and backbulge regions vulnerable to erosion and non-preservation. Retreating collisional foreland basin systems (such as those in the Mediterranean region) are often associated with large subducted slab loads, which produce narrow but very thick accumulations in the foredeep and wedge-top depozones. These foreland basin systems are characterized by very thick foredeep and wedge-top deposits, well beyond what would be expected from topographic loading alone. Changing lithospheric stiffness in collisional settings mayaffectpreservationofthebackbulgeandforebulgedepozones.Ifthesedistalforeland basin deposits are not preserved, roughly half the history of the orogenic event (as archived in the stratigraphic record) may be lost. Many foreland stratigraphic successions provide sufficient information to estimate thevelocityofmigrationoftheflexuralwavethroughtheforeland,whichmayinturnbe decomposed into propagation and shortening velocities in the thrust belt. Foreland basin subsidence curves may be inverted to produce an idealized flexural profile, from

A long‐standing problem in the geological evolution of the India‐Asia collision zone is how and where convergence between India and Asia was accommodated since collision. Proposed collision ages vary from 65 to 35 Ma, although most data sets are consistent with collision being underway by 50 Ma. Plate reconstructions show that since 50 Ma ∼2400-3200 km (west to east) of India‐Asia convergence occurred, much more than the 450-900 km of documented Himalayan shortening. Current models therefore suggest that most post‐50 Ma convergence was accommodated north of the Indus‐Yarlung suture zone. We review kinematic data and construct an updated restoration of Cenozoic Asian deformation to test this assumption. We show that geologic studies have documented 600-750 km of N‐S Cenozoic shortening across, and north of, the Tibetan Plateau. The Pamir‐Hindu Kush region accommodated ∼1050 km of N‐S convergence. Geological evidence from Tibet is inconsistent with models that propose 750-1250 km of eastward extrusion of Indochina. Approximately 250 km of Indochinese extrusion from 30 to 20 Ma of Indochina suggested by SE Asia reconstructions can be reconciled by dextral transpression in eastern Tibet. We use our reconstruction to calculate the required size of Greater India as a function of collision age. Even with a 35 Ma collision age, the size of Greater India is 2-3 times larger than Himalayan shortening. For a 50 Ma collision, the size of Greater India from west to east is ∼1350-2600 km, consistent with robust paleomagnetic data from upper Cretaceous‐Paleocene Tethyan Himalayan strata. These estimates for the size of Greater India far exceed documented shortening in the Himalaya. We conclude that most of Greater India was consumed by subduction or underthrusting, without leaving a geological record that has been recognized at the surface.

[1] The late Miocene–Pleistocene Zhada basin in southwestern Tibet provides a record of subsidence and basin formation within an active collisional thrust belt. The >800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of Tethyan strata that were previously shortened in the Himalayan fold-thrust belt. Modal sandstone petrographic data, conglomerate clast count data, and detrital zircon U-Pb age spectra indicate a transition from detritus dominated by a distal, northern source to a local, southern source. This transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin-centric. At the same time the depositional environment in the Zhada basin changed from a large, braided river to a closed-basin lake. Sedimentation in the Zhada basin was synchronous with displacement on the Qusum and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively. These observations indicate that accommodation for Zhada basin fill was produced by a combination of tectonic subsidence and damming, as midcrustal rocks were evacuated from beneath the Zhada basin in response to arc-parallel slip on crustal-scale detachment faults.

The Occurrence of cyclic patterns of sedimentation on a large scale, or abrupt changes in lithology or facies patterns in foreland basins, are most commonly attributed to tectonism. Climatic controls are invoked much less often, and geomorphic controls are rarely considered except for small-scale features. Tectonism is the first-order control on sedimentation at mountain fronts by providing accommodation space for sediment accumulation, and the requisite energy for the system to operate. However, geomorphic controls on sediment yield from source areas, transformation of sediment yield in transfer systems, and feedback mechanisms between source areas and depositional basins may be the secondary controls on sediment dispersal and accumulation near mountain fronts. Drainage basins pass through a series of stages during their evolution. Sediment flux is large during initial stages of basin evolution, allowing fans to prograde rapidly. I3ut as drainage nets in the source area expand, and valleys increase their capacity to store sediment eroded from interfluves, the quantity and caliber of the sediment load at the outlet diminishes, and fan sequences begin to fine upward. Eventually the source area drainage network will expand to its maximum size. Relief is greatest in the source area at this time, and the quantity and calibre of the sediment eroded from valley walls reaches a maximum. Dynamic equilibrium between fan and source area is attained and a period during which spontaneous incision of source area valleys, fanhead entrenchment, and depositional lobe progradation occurs The amount and size of sediment supplied to the fan reaches a maximum at this time, and fan deposits coarsen upward through this period. However, feedback relationships between fan and depositional basin limit the ability of fans to prograde basinward. Fans begin to retrograde as relief in the source area declines and storage capacity increases. Fining-upward sequences are. deposited and basinal facies encroach on the fan during this final phase. Ancient alluvial fans commonly consist of coarsening- then fining-upward stratigraphic sequences consistent with this evolutionary model, and crosssections of alluvial fan deposits normally show that fan facies stack vertically near their upland sources. A typical first order fan deposit ranges in thickness between 100 m and 250 m, but those in foreland basins near thrust margins are considerably thicker, possibly in response to increased accommodation space provided by basin subsidence, extended periods of downcutting caused by continued movement on the thrust, and to basinward translation of the source area.

Dacitic pyroclastic flow deposits within the Tecuya Formation of the San Emigdio Mountains, California, exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the San Joaquin basin. The dacitic pyroclastic deposits are part of an informal volcanic member within the Tecuya Formation, consisting of a lower dacitic unit and an upper basaltic unit. Volcanogenic facies transitions within each of these units can be traced westward, obliquely across paleoshoreline, for 15 km. Depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits. The lowermost dacitic unit consists of pyroclastic surge and fallout deposits, which overlie nonmarine alluvial-fan deposits and were produced by subaerial, phreatomagmatic eruptions. A second, magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits, and within marine, nearshore to outer-shelf settings. These pyroclastic flows, generated from subaerial vents, were high-concentration, poorly fluidized flows which traveled down paleoslope into the marine part of the basin. Upon reaching water, the hot pyroclastic flows continued to flow subaqueously, partially mixed with water, and were quenched. Transitions to cool submarine pyroclastic debris flows may have occurred. Surface transformations of the submarine pyroclastic flows/pyroclastic debris flows generated dilute pyroclastic turbidity flows, which at times became detached and continued to flow independently downslope. Progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east. A second, probably submarine, vent also produced submarine pyroclastic flow deposits in the western part of the basin. Deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge, fallout, and flow deposition. The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits within the Tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin.

[1] The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction erosion is an important process during the evolution of this margin. Turbidites derived from the uplifted forearc are the predominant sedimentary input into this trench, while pelagic sediments are subordinate. Apatite (U-Th)/He ages were obtained on 23 samples from two transects across the Sierra Madre del Sur (Acapulco and Puerto Escondido) and reveal slow cooling during the Miocene. (U-Th)/He ages range between ∼25 and 8 Ma and correlate inversely with elevation. Long-term erosional exhumation rates inferred from these ages range from 0.11 to 0.33 km/m.y., with higher rates in the range core, and suggest that the Sierra Madre del Sur has been a slowly decaying mountain range, since at least the early Miocene. Apparent Miocene-Pliocene sedimentation (“preservation”) rates in the Acapulco trench derived from Deep Sea Drilling Project data are about an order of magnitude smaller than the Miocene forearc erosion rates estimated from (U-Th)/He ages, suggesting that the terrigenous input to the trench was almost entirely recycled via subduction erosion, at least during the Miocene. The Miocene subducted flux per unit length of the margin is about 30 km3/(km m.y.), or a subducted volume per unit time of 44 × 103 km3/m.y., when integrated over the length of the trench.

Hummocky cross-stratification (HCS) occurs in two Pliocene progradational nearshore-marine to fluvial sequences in the town of Guardavalle, Calabria (southern Italy). The HCS is notable because of its shallow depth of deposition ( 2-5 m) and its coarse grain size (up to coarse sand). Individual units of HCS consist of, in ascending order, three subfacies which are characterized by distinctive internal structure: a massive subfacies; a centimeter-scale laminated (meso-laminated) subfacies; and a millimeterscale laminated (micro-laminated) subfacies. The massive subfacies consists of normally graded, coarse to fine sand ( = 1.98-1.85), in beds a few cm to 25 cm thick. These beds have flat to broadly undulatory bases and hummocky-swaley tops. The mesolaminated subfacies consists of sets of 0.8- to 1-cm-thick laminae of normally graded, medium to fine sand ( = 2.3) that drape the underlying massive subfacies, thicken gradually toward swales, and then flatten upward. The micro-laminated subfacies comprises sets of several, 2- to 8-ram-thick, planar to slightly undulatory laminae that are characterized by mica-rich and mica-poor zones. Individual micro-laminae are inversely graded, from silt and very fine sand at their hases to fine to medium sand at their tops ( = 2.84-2.95). The upper few tenths of individual lamin e are highly enriched (up to 56%) in biotite grains, most of which are aligned parallel to the lamination. The most likely mode of deposition of the massive and meso-laminated subfacies was rapid settling from suspension from increasingly unsteady flows. The bulk of the sediment was probably imported to the depositional site by unidirectional flows during major storms. Hummocky-swaley laminae did not develop during deposition of the massive subfacies because of the high rate of suspended-load fallout (), which probably was controlled by the relatively coarse grain size. The grains composing the microlaminae are in dispersive, not hydraulic, equivalence. The thickness/grain-size relationships within micro-laminae satisfy predictions of grain-flow theory and indicate that the laminae were produced by collapse of traction carpets under the influence of currents on the order of 80-150 cm/s. Deposition of the Guardavalle HCS took place in four stages: 1) initial plane bed; 2) growth of hummocky bedforms while was very high, probably under the influence of combined orbital and unidirectional currents; 3) draping of the hummocky bedforms by meso-laminae during a period of moderately high , high orbital velocity, but lower unidirectional-current velocity; and 4) reworking of the seafloor by powerful orbital currents under conditions of relatively low and minor unidirectional currents. Preservation of the Guardavalle HCS in a very shallow-marine environment was promoted by the microtidal, storm-wave-dominated chara ter of the Pliocene Ionian coast of Calabria.

Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wall) and Lesser Himalayan (footwall) metasedimentary rocks. ϵ Nd (0) values for lower Lesser Himalayan rocks typically range from −20 to −26, whereas Greater Himalayan rocks usually have ϵ Nd (0) values of −19 to −12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-to-the-south ductile shear zone superimposed on this boundary, confirming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range.

Newly released reflection seismic and borehole data, combined with sedimentological, provenance, and biostratigraphic data from Upper Cretaceous–Paleocene strata in the proximal part of the Cordilleran foreland-basin system in Utah, establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin. During Campanian time, coarse-grained sand and gravel were derived from the internally shortening Charleston-Nebo salient of the Sevier thrust belt. A rapid, regional Campanian progradational event in the distal foreland basin (>200 km from the thrust belt in

[1] Recent research in the Himalayan fold-thrust belt provides two new sets of observations that are crucial to understanding the evolution of the Himalayan-Tibetan orogenic system. First, U-Pb zircon ages and Sm-Nd isotopic studies demonstrate that (1) Greater Himalayan medium- to high-grade metasedimentary rocks are much younger than true Indian cratonic basement; and (2) these rocks were tectonically mobilized and consolidated with the northern margin of Gondwana during early Paleozoic orogenic activity. These observations require that Greater Himalayan rocks be treated as supracrustal material in restorations of the Himalayan fold-thrust belt, rather than as Indian cratonic basement. In turn, this implies the existence of Greater Himalayan lower crust that is not exposed anywhere in the fold-thrust belt. Second, a regional compilation of shortening estimates along the Himalayan arc from Pakistan to Sikkim reveals that (1) total minimum shortening in the fold-thrust belt is up to ∼670 km; (2) total shortening is greatest in western Nepal and northern India, near the apex of the Himalayan salient; and (3) the amount of Himalayan shortening is equal to the present width of the Tibetan Plateau measured in an arc-normal direction north of the Indus-Yalu suture zone. This information suggests that a slab of Greater Indian lower crust (composed of both Indian cratonic lower crust and Greater Himalayan lower crust) with a north-south length of ∼700 km may have been inserted beneath the Tibetan crust during the Cenozoic orogeny. We present a modified version of the crustal underthrusting model for Himalayan-Tibetan orogenesis that integrates surface geological data, recent results of mantle tomographic studies, and broadband seismic studies of the crust and upper mantle beneath the Tibetan Plateau. Previous studies have shown that incremental Mesozoic and early Cenozoic shortening had probably thickened Tibetan crust to ∼45–55 km before the onset of the main Cenozoic orogenic event. Thus, the insertion of a slab of Greater Indian lower crust with maximum thickness of ∼20 km (tapering northward) could explain the Cenozoic uplift of the Tibetan Plateau. The need for Tibetan crust to stretch laterally as the Greater Indian lower crust was inserted may explain the widespread east-west extension in the southern half of the Plateau. Our reconstruction of the Himalayan fold-thrust belt suggests that Indian cratonic lower crust, of presumed mafic composition and high strength, should extend approximately halfway across the Tibetan Plateau, to the Banggong suture. From there northward, we predict that the Tibetan Plateau is underlain by more felsic, and therefore weaker, lower crust of Greater Himalayan affinity. Two slab break-off events are predicted by the model: the first involved Neotethyan oceanic lithosphere that foundered ∼45–35 Ma, and the second consisted of Greater Indian lithosphere (most likely composed of Greater Himalayan material) that delaminated and foundered ∼20–10 Ma. Asthenospheric upwelling associated with the break-off events may explain patterns of Cenozoic volcanism on the Tibetan Plateau. Although the model predicts a northward migrating topographic front due solely to insertion of Greater Indian lower crust, the actual uplift history of the Plateau was complicated by early-middle Tertiary shortening of Tibetan crust.

The rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen. This rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface. The rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening. This rate is controlled by the rates of mass accretion to the orogenic wedge and erosion. The orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction (or convergence or shortening). In orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections, it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge. In addition, orogenic wedges may accelerate solely in response to a reduction in taper.

Regional variations in sediment thickness, internal structures, average elevation, and Bouguer gravity define a four-component foreland basin system adjacent to the Central Andes. In the most proximal part of the foreland basin system, the eastern Subandean zone and westernmost Chaco Plain, 1–3 km of Cenozoic deposits overlies active folds and thrusts of the frontal Andean orogenic wedge. These wedge-top deposits pass cratonward into a foredeep depozone containing a 3–4-km-thick sedimentary prism that tapers toward (and locally pinches out against) a broad-wavelength forebulge in the central-eastern Chaco Plain. The forebulge is underlain by Precambrian–Mesozoic rocks and is largely covered by a thin veneer of Quaternary alluvium. East of the forebulge, a thin (0.5 km) saucer-shaped accumulation of sediment beneath the Pantanal Wetland represents a back-bulge depozone. Ancient counterparts of these four depozones can be identified in the Central Andes, suggesting that modern basin architecture is the result of continuous, eastward migration of the coupled orogenic wedge and foreland basin system since the Late Cretaceous–Paleocene.

Problems with applying critical taper models to ancient orogenic wedges are overcome in the Late Cretaceous–late Paleocene Sevier orogenic wedge in Utah and Wyoming by a symptomatic approach in which wedge behavior is inferred from the time-space distribution of thrust faulting, erosion, and synorogenic sediment accumulation in association with the orogenic wedge. In turn, the causes of wedge behavior are interpreted in terms of features that are known about the Sevier wedge, such as the locations of major decollements and the lithologic constituents of major thrust sheets. From Coniacian through late Paleocene time (∼35 m.y.), basement and cover rocks in the Sevier wedge were shortened by ∼100 km in three major and one minor events. An overall eastward progression of thrusting was punctuated by several episodes of out-of-sequence and hinterland vergent thrusting. Over the long term, wedge taper was controlled by internal wedge strength, initial taper, and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy. Critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge. While the basal decollement was mainly in basement rocks, the wedge was highly tapered and thrust spacing was relatively close (∼15 km). Beginning ca. 75 Ma, the basal decollement ramped upward and eastward into weak Cambrian shale and Jurassic salt, thrust spacing increased to 25–45 km, and wedge taper decreased drastically. The highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements, whereas the frontal, lower-taper part of the wedge carried sedimentary rocks above extremely weak (salt and shale) decollements. Through time, internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks; the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time, both by strain softening (beneath the rear of the wedge) and by propagating along extremely weak rocks (beneath the front of the wedge). Provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long-term history of the Sevier wedge, as an increasing proportion of the erosional surface became occupied by resistant Precambrian quartzite and basement rocks. This would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope, which would have helped the wedge to remain at critical to supercritical taper. The Sevier wedge also exhibits several shorter-term ( 10 km, and (3) was subjected to regional erosion (marked by major unconformities) that ultimately caught up with tectonic thickening and caused the wedge to stall. This cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag-time between wedge thickening (and uplift) and wedge erosion. Erosion eventually catches up to the rate of uplift, the wedge stalls, and the locus of deformation shifts to the rear of the wedge in order to rebuild taper. This analysis suggests that orogenic wedges continually deform and “find a way” to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists. The actual taper value may change substantially through time as lithostratigraphy, internal strength, upper surface durability, and presence/absence of weak potential stress guides conspire to maintain a critical taper.

Widespread uppermost Miocene conglomerate and sandstone along the Apenninic-Maghrebian orogenic belt in the central Mediterranean region cannot be explained as a result of the Messinian base-level falls. Along the Ionian coast of Calabria, southern Italy, these rocks were deposited in marine fan deltas and rest in angular unconformity or disconformity upon the internal part of the Calabrian accretionary wedge. We propose that the upper Messinian deposits were produced by internal shortening of the Calabrian accretionary wedge as it compensated for the decrease in upper surface slope caused by flexural rebound as the ∼3.4-km-thick Ionian water mass evaporated. Latest Miocene-Pliocene marine inundation reloaded the basin, restored the wedge to a critical state, and caused the rear part of the wedge again to become tectonically stable. This isostatically driven mechanism could explain widespread latest Messinian thrust faults and coarse siliciclastic deposits along much of the Apenninic-Maghrebian orogen.

Most of the regional shortening in the type area of the Sevier orogenic belt in central Utah was accommodated by displacement on the Canyon Range (Neocomian-Aptian), Pavant (Aptian-Albian), Paxton (Cenomanian-Campanian), and Gunnison (late Campanian–Paleocene) thrust systems. Inception of each thrust system generated synorogenic sediment associated with frontal thrust-tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland. The Sevier culmination, a large antiformal duplex cored by crystalline basement rocks, developed during Paxton and Gunnison thrusting west of and structurally beneath the Canyon Range and Pavant thrusts. Growth of the Sevier culmination was coeval with reactivation of the Canyon Range and Pavant thrust systems and produced a second culmination in Proterozoic–Lower Cambrian rocks in the Canyon Range. These structural highs provided much of the sediment to the adjacent foreland basin from late Cenomanian to late Paleocene time, and may have helped to maintain critical taper in the thrust belt.

The Eocene to lower Miocene fill of the southern San Joaquin basin contains three complete depositional sequences--the Tejon, San Emigdio, and Pleito--each of which corresponds to a formal formation. The Tejon sequence (lower to middle Eocene) is marine and incorporates nearshore, shelf, slope, and basinal deposits. The San Emigdio (upper Eocene to lower Oligocene) and Pleito (upper Oligocene) sequences intertongue eastward with alluvial-fan deposits of the Tecuya Formation. The lower part of the San Emigdio sequence was deposited by a westward-prograding Gilbert-type delta. The upper part of the San Emigdio sequence and lower part of the Pleito sequence were deposited by a system of shelf fan-deltas that prograded at least 10 km to the west. The middle and upper parts of the Pleito sequence were deposited by a slope fan-delta in relatively deep (hundreds of meters) water. Regional transgression during the early Eocene initiated deposition in the southern San Joaquin basin. The lower San Emigdio Gilbert-type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 Ma. Relative highstand deposits in the San Emigdio and Pleito Formations consist of widespread progradational shallow-marine and nonmarine facies. The Eocene to early Miocene tectonic history of the southern San Joaquin basin included three distinct periods of increasingly intense activity. Rapid early to middle Eocene subsidence of the basin was associated with the emplacement of the Salinian block. Late Eocene to early Oligocene uplift and subsidence occurred in two discrete pulses, and a seismically(?) generated submarine-slumping episode triggered the change from shelf fan-deltas to slope fan-deltas. During the late Oligocene to early Miocene, the southern San Joaquin basin was disrupted by major uplift, proximal volcanism, and syndepositional faulting, as the Mendocino triple junction migrated past the study area, and San Andreas fault-related tectonism was initiated. In general, local tectonic events, rather than eustatic ea level events, seem to have exerted the predominant control on middle Cenozoic sedimentation in the southern San Joaquin basin.

The middle Tertiary (Oligocene-Early Miocene) strata of the San Emigdio Range in southern California were deposited along a shoreline oriented north-northeast to south-southwest, adjacent to a tectonically active highland that formed the southern margin of the San Joaquin Basin. Abundant storm deposits (sandstone and conglomerate) are preserved in the San Emigdio, Pleito, and lower Temblor Formations. Extensive lateral exposures allow recognition of fair-weather and storm-dominated facies in contemporaneous nearshore, inner-shelf, and outer-shelf deposits. The nearshore deposits consist of coarse-grained progradational sequences that are similar to those described by Clifton (1981) in Miocene rocks of the nearby Caliente Range. Each complete sequence consists of, in ascending order, a basal transgressive lag, followed by inner-shelf, surf-zone, longshore-trough, and beach-foreshore facies. The inner-shelf deposits consist of fair- and foul-weather deposits. The former deposits comprise massive, locally pebbly, very fine-grained sandstone, whereas the latter deposits consist of alternating intervals of cross-stratified (landward-oriented) conglomerate and hummocky cross-stratified fine-grained sandstone. The outer-shelf deposits are composed of massive, very fine-grained to silty sandstone, and sequences of mollusk beds and hummocky cros -stratified sandstone. The massive outer-shelf sandstones were deposited during periods of dominantly fair weather. The sequences of hummocky cross-stratified sandstone were deposited during and immediately after storms. The outer-shelf storm deposits apparently were preserved on an event-by-event basis, whereas the inner-shelf storm deposits were preserved in amalgamated packages. Unmodified storm deposits are rare in the nearshore deposits, owing to the reworking capabilities of high-energy nearshore processes. Reconstructions of wave conditions based on Airy wave theory, paleogeographic constraints, and storm facies suggest that peak storm waves were more than 5 m high, with periods of 7-9 seconds. The study area was situated at the end of a long fetch, in a favorable position to receive the full brunt of northwesterly wind-driven waves.